1. Field of the Invention
This invention relates to polymer and metal composite implantable medical devices, such as stents.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of an endoprosthesis. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty or valvuloplasty) with apparent success.
Stents have been made of many materials including metals and polymers. Polymer materials include both biostable and biodegradable polymer materials. Metallic stents are typically formed from biostable metals. However, bioerodable metal stents have been described. U.S. Pat. No. 6,287,332 B1 to Bolz et al., U.S. Pat. Appl. Pub. No. 2002/0004060 A1 to Heublein et. al. The cylindrical structure of stents is typically composed of a scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or planar films or sheets of material rolled into a cylindrical shape. In addition, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier. The polymeric carrier can include an active agent or drug. Furthermore, the pattern that makes up the stent allows the stent to be radially expandable and longitudinally flexible. Longitudinal flexibility facilitates delivery of the stent and radial rigidity is needed to hold open a bodily lumen. The pattern should be designed to maintain the longitudinal flexibility and radial rigidity required of the stent.
A number of techniques have been suggested for the fabrication of stents from tubes and planar films or sheets. One such technique involves laser cutting or etching a pattern onto a material. A pattern may be formed on a planar film or sheet of a material which is then rolled into a tube. Alternatively, a desired pattern may be formed directly onto a tube. Other techniques involve forming a desired pattern into a sheet or a tube via chemical etching or electrical discharge machining. Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. No. 5,922,005 to Richter and U.S. Pat. No. 5,906,759 to Richter.
The first step in treatment of a diseased site with a stent is locating a region that may require treatment such as a suspected lesion in a vessel, typically by obtaining an X-Ray image of the vessel. To obtain an image, a contrast agent which contains a radio-opaque substance such as iodine is injected into a vessel. Radio-opaque refers to the ability of a substance to absorb X-Rays. The X-ray image depicts a profile of the vessel from which a physican can identify a potential treatment region. The treatment then involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involved compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn allowing the stent to self-expand. The stent may be visualized during delivery and deployment using X-Ray imaging if it contains radio-opaque materials. By looking at the position of stent with respect to the treatment region, the stent may be advanced with the catheter to a location. After implantation of the stent additional contrast agent may be injected to obtain an image of the treated vessel. There are several desirable properties for a stent to have that greatly facilitate the delivery, deployment, and treatment of a diseased vessel.
Longitudinal flexibility is important for successful delivery of the stent. In addition, radial strength is vital for holding open a vessel. Also, as the profile of a stent decreases, the easier is its delivery, and the smaller the disruption of blood flow. Additionally, in order to visualize a stent during deployment it is also important for a stent to include at least some radio-opaque materials. Furthermore, it is also desirable for a stent to be bioeroable. Many treatments utilizing stents require the presence of a stent in the vessel for between about six and twelve months. Stents fabricated from biodegradable polymers may be configured to completely erode after the clinical need for them has ended.
Although current biodegradable polymer-fabricated stents, biostable metal stents, bierodable metal stents, and polymer-coated metal stents each have certain advantages, they also possess potential shortcomings. Biodegradable polymer-fabricated stents may be configured to degrade after they are no longer needed and also possess a desired degree of flexibility. However, in order to have adequate mechanical strength, such stents require significantly thicker struts than a metallic stent, which results in a larger profile. Inadequete radial strength may contribute to relatively high incidence of recoil of polymer stents after implantation into vessels. In addition, biodegradable polymers, unlike metals, are not radio-opaque which makes visualization of a stent difficult during delivery and after deployment. Moreover, although biostable metallic stents possess favorable mechanical properties, are radio-opaque, and have smaller profiles than polymer-fabricated stents, they are not bioerodable. Bioerodable metallic stents tend to erode too fast, resulting in complete or nearly complete bioerosion before the end of a treatment time. Therefore, there is a present need for stents that possess more of the favorable properties of polymers and metals.